Rebuilding a 1982 Brougham Motorhome on an Econoline E350 Chassis with a Ford 400 motor.

Motor was replaced in 1998 (by previous owner) with a remanufactured motor and has 17,000 miles on it.

It cranked fine and we have replace the fuel tank, rebuilt the carbeurator, new spark plug wires and plugs, distributor cap, and completely blown out and cleaned fuel lines.

It cranks and runs perfectly smoothly - sound perfect - however as it moves it only had enough power to move on completely flat land (about 20 mph) nothing more.

Verified timing and dropped mufler to look for for obstruction - nothing.

Has my guy - a 40 year shade tree mechanic (and all his buddies) absolutely stumped. This age and type is in his wheelhouse and he is out of options - any ideas?

Planning to convert it to deisel but wanted to run this motor for a while to make sure I like the RV before spending that money.

Powerless in Florida